diff options
Diffstat (limited to 'Documentation/locking/locktypes.rst')
| -rw-r--r-- | Documentation/locking/locktypes.rst | 42 |
1 files changed, 18 insertions, 24 deletions
diff --git a/Documentation/locking/locktypes.rst b/Documentation/locking/locktypes.rst index 1b577a8bf982..80c914f6eae7 100644 --- a/Documentation/locking/locktypes.rst +++ b/Documentation/locking/locktypes.rst @@ -10,7 +10,7 @@ Introduction ============ The kernel provides a variety of locking primitives which can be divided -into two categories: +into three categories: - Sleeping locks - CPU local locks @@ -164,14 +164,14 @@ by disabling preemption or interrupts. On non-PREEMPT_RT kernels local_lock operations map to the preemption and interrupt disabling and enabling primitives: - =========================== ====================== - local_lock(&llock) preempt_disable() - local_unlock(&llock) preempt_enable() - local_lock_irq(&llock) local_irq_disable() - local_unlock_irq(&llock) local_irq_enable() - local_lock_save(&llock) local_irq_save() - local_lock_restore(&llock) local_irq_save() - =========================== ====================== + =============================== ====================== + local_lock(&llock) preempt_disable() + local_unlock(&llock) preempt_enable() + local_lock_irq(&llock) local_irq_disable() + local_unlock_irq(&llock) local_irq_enable() + local_lock_irqsave(&llock) local_irq_save() + local_unlock_irqrestore(&llock) local_irq_restore() + =============================== ====================== The named scope of local_lock has two advantages over the regular primitives: @@ -211,9 +211,6 @@ raw_spinlock_t and spinlock_t raw_spinlock_t -------------- -raw_spinlock_t is a strict spinning lock implementation regardless of the -kernel configuration including PREEMPT_RT enabled kernels. - raw_spinlock_t is a strict spinning lock implementation in all kernels, including PREEMPT_RT kernels. Use raw_spinlock_t only in real critical core code, low-level interrupt handling and places where disabling @@ -247,7 +244,7 @@ based on rt_mutex which changes the semantics: Non-PREEMPT_RT kernels disable preemption to get this effect. PREEMPT_RT kernels use a per-CPU lock for serialization which keeps - preemption disabled. The lock disables softirq handlers and also + preemption enabled. The lock disables softirq handlers and also prevents reentrancy due to task preemption. PREEMPT_RT kernels preserve all other spinlock_t semantics: @@ -353,14 +350,14 @@ protection scope. So the following substitution is wrong:: { local_irq_save(flags); -> local_lock_irqsave(&local_lock_1, flags); func3(); - local_irq_restore(flags); -> local_lock_irqrestore(&local_lock_1, flags); + local_irq_restore(flags); -> local_unlock_irqrestore(&local_lock_1, flags); } func2() { local_irq_save(flags); -> local_lock_irqsave(&local_lock_2, flags); func3(); - local_irq_restore(flags); -> local_lock_irqrestore(&local_lock_2, flags); + local_irq_restore(flags); -> local_unlock_irqrestore(&local_lock_2, flags); } func3() @@ -379,14 +376,14 @@ PREEMPT_RT-specific semantics of spinlock_t. The correct substitution is:: { local_irq_save(flags); -> local_lock_irqsave(&local_lock, flags); func3(); - local_irq_restore(flags); -> local_lock_irqrestore(&local_lock, flags); + local_irq_restore(flags); -> local_unlock_irqrestore(&local_lock, flags); } func2() { local_irq_save(flags); -> local_lock_irqsave(&local_lock, flags); func3(); - local_irq_restore(flags); -> local_lock_irqrestore(&local_lock, flags); + local_irq_restore(flags); -> local_unlock_irqrestore(&local_lock, flags); } func3() @@ -439,11 +436,9 @@ preemption. The following substitution works on both kernels:: spin_lock(&p->lock); p->count += this_cpu_read(var2); -On a non-PREEMPT_RT kernel migrate_disable() maps to preempt_disable() -which makes the above code fully equivalent. On a PREEMPT_RT kernel migrate_disable() ensures that the task is pinned on the current CPU which in turn guarantees that the per-CPU access to var1 and var2 are staying on -the same CPU. +the same CPU while the task remains preemptible. The migrate_disable() substitution is not valid for the following scenario:: @@ -456,9 +451,8 @@ scenario:: p = this_cpu_ptr(&var1); p->val = func2(); -While correct on a non-PREEMPT_RT kernel, this breaks on PREEMPT_RT because -here migrate_disable() does not protect against reentrancy from a -preempting task. A correct substitution for this case is:: +This breaks because migrate_disable() does not protect against reentrancy from +a preempting task. A correct substitution for this case is:: func() { @@ -506,7 +500,7 @@ caveats also apply to bit spinlocks. Some bit spinlocks are replaced with regular spinlock_t for PREEMPT_RT using conditional (#ifdef'ed) code changes at the usage site. In contrast, usage-site changes are not needed for the spinlock_t substitution. -Instead, conditionals in header files and the core locking implemementation +Instead, conditionals in header files and the core locking implementation enable the compiler to do the substitution transparently. |